From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt1-f196.google.com ([209.85.160.196]:46000 "EHLO mail-qt1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727475AbfBQXnD (ORCPT ); Sun, 17 Feb 2019 18:43:03 -0500 Subject: Re: [LSF/MM TOPIC] More async operations for file systems - async discard? References: <92ab41f7-35bc-0f56-056f-ed88526b8ea4@gmail.com> <20190217210948.GB14116@dastard> From: Ric Wheeler Message-ID: <46540876-c222-0889-ddce-44815dcaad04@gmail.com> Date: Sun, 17 Feb 2019 18:42:59 -0500 MIME-Version: 1.0 In-Reply-To: <20190217210948.GB14116@dastard> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs To: Dave Chinner Cc: lsf-pc@lists.linux-foundation.org, linux-xfs , linux-fsdevel , linux-ext4 , linux-btrfs , linux-block@vger.kernel.org On 2/17/19 4:09 PM, Dave Chinner wrote: > On Sun, Feb 17, 2019 at 03:36:10PM -0500, Ric Wheeler wrote: >> One proposal for btrfs was that we should look at getting discard >> out of the synchronous path in order to minimize the slowdown >> associated with enabling discard at mount time. Seems like an >> obvious win for "hint" like operations like discard. > We already have support for that. blkdev_issue_discard() is > synchornous, yes, but __blkdev_issue_discard() will only build the > discard bio chain - it is up to the caller to submit and wait for it. > > Some callers (XFS, dm-thinp, nvmet, etc) use a bio completion to > handle the discard IO completion, hence allowing async dispatch and > processing of the discard chain without blocking the caller. Others > (like ext4) simply call submit_bio_wait() to do wait synchronously > on completion of the discard bio chain. > >> I do wonder where we stand now with the cost of the various discard >> commands - how painful is it for modern SSD's? > AIUI, it still depends on the SSD implementation, unfortunately. I think the variability makes life really miserable for layers above it. Might be worth constructing some tooling that we can use to validate or shame vendors over - testing things like a full device discard, discard of fs block size and big chunks, discard against already discarded, etc. Regards, Ric